The demand for high speed telecommunications transmission has grown rapidly in later years. In order to provide such high speed telecommunications transmission, wide bandwidths including high frequencies are being used. Ordinary copper wires, originally intended for analogue telephony, are upgraded to Digital Subscriber Lines DSL, using various transmission techniques. The analogue bandwidth of about 4 kHz, is often increased to several MHz. However, the handling of high bandwidths puts several requirements on the transmission equipment in terms of frequency response, linearity, distortion, noise etc and especially power consumption.
Various transmission and encoding techniques are used for the abovementioned upgrading, e.g. Discrete Multi Tone DTM, for handling ADSL transmission on ordinary copper wires. Generally, all DSL transmission methods, regardless of type (ADSL, VDSL, HDSL, SHDSL etc.), involve a transmission design based on certain digital parts, e.g. DSPs, CODECs and analogue front-end parts with linear broadband amplifiers for both the transmit and receive direction.
The abovementioned amplifiers are typically based on traditional broadband amplifiers using Class A, Class A/B, Class B or even Class G amplifier principles. These amplifier classes are all characterised by having a “built-in” power dissipation because of the design principle itself. The amplifiers have ordinary push-pull transistor coupling with a constant bias current in order to reduce e.g. cross over distortion.
The DSL transmission requires typically 100 mW of power transmitted to the line. In order to handle this transmission power, the ordinary amplifiers dissipate up to approximately 800 mW or even 900 mW. The Class G amplifier, using two supply voltages, may reduce the dissipation to approximately 600 mW or even less. Still there is currently a substantial power dissipation around five or six times higher than the required power transferred to the line.
This very high power dissipation and consumption leads to an expensive and bulky implementation, which becomes increasingly more challenging to handle in Central Office development. The amount of broadband subscribers are also expected to increase rapidly in the coming years.
In the international patent application with publication number WO 98/19391 is described the use of a Class D amplifier for the audio frequency range. The Class D amplifier is a Pulse Modulation Amplifier PMA and has among others the advantages that the power dissipation is very low, low complexity and good fidelity. The PMA disclosed in the WO-application includes a pulse modulator including a power amplifier stage. The power amplifier output has one or more negative feedback loops to preamplifiers preceding the modulator. In one disclosed embodiment the pulse modulator is a self oscillating modulator. The PMA is connected to a loudspeaker and has a low output impedance.